Static variables are needed to access them without creating an instance of the class. But why do we need a static class?

Answer 1, authority 100%

Only a nested class can be a static class in java.
If a class is marked static then it behaves like a regular class.

for example, there is class A, nested static class B and nested (non-static) class C:

public class A {
...
 static public class B {
 }
 public class C {
 }
}

and we want to create instances of these classes in “external” code

public class Test {
  public static void main (String [] args) {
    A a = new A (); // normal class
    A.B b = new A.B (); // static nested class
    A.C c = a.new C (); // nested class associated with instance A
    // A.C c = new A.C (); // syntax error (won't compile)
  }
}

or inside static methods of class A

public class A {
...
  static public class B {
  }
  public class C {
  }
  public static void main (String [] args) {
    A a = new A (); // normal class
    A.B b = new A.B (); // static nested class
    A.C c = a.new C (); // nested class associated with instance A
    // A.C c = new A.C (); // syntax error (won't compile)
  }
  public static void test () {
    A a = new A (); // normal class
    A.B b = new A.B (); // static nested class
    A.C c = a.new C (); // nested class associated with instance A
    // A.C c = new A.C (); // syntax error (won't compile)
  }
}

In my opinion, the use of a static class may be appropriate as a small class, which in its meaning is closely related to the “main” outer class.

For example:

public class Tree {
  static public class Node {
  }
}

In this situation, you can also move the nested class into a regular one and move both classes into a separate package.

The only difference between a nested static class and a regular class that I see is a more forgiving attitude to visibility
methods and fields between the nested class and its outer class.

For example:

public class A {
  private void privateMethod () {
   B b = new B ();
   b.privateMethod (); // there is access to private methods / fields
  }
  static public class B {
   private void privateMethod () {
     A a = new A ();
     a.privateMethod (); // there is access to private methods / fields
   }
  }
}

Link to documentation:
https://docs.oracle.com/javase/tutorial/java/javaOO /nested.html

Answer 2, authority 48%

Basically so that you can create nested classes, objects of which can be created without creating an instance of the class in which it lies.

Answer 3

“Only a nested class can be a static class in java” – there are nested (static ) and internal ones, the difference, as in the usual plan, static has access only to static fields